Monocytes and their tissue counterparts, macrophages, have multiple functions in host defense and may play a role against tumors when activated by specific agents to acquire cytolytic or cytostatic capabilities. We have investigated the metabolic changes associated with the acquisition or the expression of cytotoxicity upon in vitro activation by biological response modifiers (BRMs). Peritoneal macrophages from responsive strains of mice, C57BL/6 or C3H/HeN, treated with interferon Gamma (IFN Gamma) or LPS, became cytotoxic against tumor targets and displayed an increased intracellular content of the active methyl donor S-adenosylmethionine (SAM). Similarly treated macrophages from the genetically deficient strain of mice, C3H/HeJ, failed to become cytotoxic and had an unchanged SAM content. Turnover studies indicated that the increased SAM content in activated cells was associated with a decreased rate of utilization together with an unmodifed synthetic rate. We concluded that the observed changes in SAM metabolism are closely associated with the induction of cytotoxicity in macrophages, and may reflect specific changes in SAM-mediated reactions, including transmethylations. In human monocytes we found that treatment with IFN Gamma or lymphokine preparations containing MAF activity induced a higher ability to secrete superoxide anion (O2), a potent antimicrobial agent. However, we failed to correlate this observation with the expression of cytotoxic activity by monocytes, since only MAF-treated cells acquired cytolytic capacity against adherent tumor targets in a long-term cytotoxicity assay. These results indicate a requirement for other mechanisms in monocyte-mediated cytotoxicity. These studies contribute to understanding the molecular mechanisms of monocyte and macrophage functions, and may help in the rational development of strategies for modulating cellular activities.